Among the myosin super-family of motor proteins, myosin II is the sole filament forming class. Myosin IIs are hexamers containing 2 identical heavy chains and 2 pairs of light chains, the essential light chain (ELC) and the regulatory light chain (RLC) and a long coiled-coil dimerization domain, S2, that extends into the LMM filament forming domain. Myosin isoforms that are "regulated" depend either on RLC phosphorylation, e.g., arthropod, smooth muscle and nonmuscle myosins, or Ca2+ binding, e.g., molluscan striated muscles. Myosin based regulation has in common a dependency on intramolecular interactions between the two myosin heads. This interaction was visualized in smooth muscle myosin and recently in arthropod thick filaments suggesting that a head-head interaction and a conformational change to a compact structure is a general feature of myosin II regulation. Biochemistry and modeling studies have identified a number of the features of the conformational change among which are torsional motions about the S2 alpha-helices that are dependent on the length of S2. These torsional motions could affect other myosin functions, including double headed interactions with the actin filament and processive motion. This grant application proposes to investigate structural and functional features of regulation in smooth muscle myosin that have a strong dependency on the length of the coiled-coil domain. Myosin V, a cytoplasmic myosin also forms a compact inhibited conformation with some similarity to that of myosin II, but with an interaction between the myosin heads and the cargo binding domain. Both myosin II and myosin V bind actin when in the inhibited conformation, but with very different affinities. While myosin V is widely identified as a processive motor, which requires simultaneous actin binding by both heads, simultaneous binding of smooth muscle myosin by both heads is controversial. The inhibited states of both myosin II and myosin V have common underlying principles. This project seeks answers to these and other aspects of two headed motor function and regulation. Finally, inhibited conformations in conventional kinesin also involve an interaction between the cargo binding domains and the kinesin heads. This conformation may have structural and functional similarities with inhibited conformations of myosin and will also be investigated.